This addresses PHPBB-13446 by adding account-security email notifications when a user changes their password or email address from the UCP.

What it does

• Password change – sends a confirmation email to the account holder's address on file, advising them to reset their password and contact the administrator if they did not make the change.
• Email change – sends a notification to the previous email address whenever the account email is changed, in both the immediate and activation-required paths. phpBB's existing new-email verification (account deactivation + reactivation via the activation link sent to the new address) is unchanged and continues to serve as the verification step.

Both emails are gated by the board email_enable setting and follow the existing direct-messenger pattern already used for account-activation and password-reset mails, via new templates password_changed.txt and email_changed.txt.

Notes for reviewers

• The notifications are not behind a separate admin toggle, consistent with phpBB's other non-optional account emails (activation, forgot-password). Happy to add a config setting + migration if preferred.

Checklist:

• Correct branch: master for new features; 3.3.x for fixes
• Tests pass
• Code follows coding guidelines: master and 3.3.x
• Commit follows commit message format

Tracker ticket:

https://tracker.phpbb.com/browse/PHPBB-13446